A new study from the US suggests that travel is linked to a higher risk of developing a deep vein thrombosis (DVT), also known as a venous thromboembolism (VTE).

The study was the work of corresponding author Dr Divay Chandra, from the Harvard School of Public Health (HSPH) and other colleagues from HSPH and Brigham and Women’s Hospital, and Harvard Medical School, all in Boston, Massachusetts. They have published their findings in the 4 August issue of Annals of Internal Medicine.

The researchers noted that although VTE linked to travel has become a public health issue, the evidence linking the two is “surprisingly contradictory”, so they set out to review the research to date and find out what the travel-related risk of VTE might be, whether it depended on the length of travel, and what might explain the contradictory evidence.

For the study they searched well known medical article databases, asked other scientists working in this field, and searched the reference lists of relevant research papers. They extended their search to include all languages.

When they collected together all their sources, they then selected only those studies that had investigated the links between any type of travel and VTE and had compared travellers and non-travellers.

Two independent researchers then reviewed the selected studies, and extracted data on patient characteristics, estimates of risk, and certain quality variables.

They found 14 studies from a total of 1,560 that met their criteria. This gave them a total of 4,055 cases of VTE.

Chandra and colleagues then pooled the results and using meta-analytical statistical tools they reviewed the data on the cases as if they had come from one giant study.

The analysis showed that:

  • Compared with non-travellers, the overall relative risk of a traveller getting a VTE was 2.0.

  • The reason the studies showed contradictory results was because of the way they selected their control subjects: ie some included subjects who had been referred for VTE evaluation while others did not.

  • Excluding the studies that had used controls who had been referred for VTE evaluation, raised the relative risk of getting a VTE to 2.8 compared to non-travellers.

  • Risk of getting a VTE went up 18 per cent for every 2-hour increase in travel duration by any means of transport.

  • But for air transport the relative risk went up 26 per cent for every 2-hour increase in travel duration.

The authors pointed out that the studies only covered Western countries, and although they suspect the results would also be true of non-Western populations this would still have to be confirmed with other studies before they could be sure.

The journal editors noted that:

“By excluding studies with control participants who had a different risk for VTE than the source population for the case-patients, the authors clarified a confusing body of evidence.”

The authors concluded that their findings provided the “strongest evidence to date of the presence and magnitude of association between travel and VTE”.

They also said research needs to be done on whether doing simple and inexpensive things like encouraging travellers to walk around and drink more water makes a difference, particularly on longer journeys.

“Travel and Risk for Venous Thromboembolism.”
D. Chandra, E. Parisini and D. Mozaffarian
Ann Intern Med 4 August 2009; Volume 151 Issue 3.

Written by: Catharine Paddock, PhD